High Sun Tangent Arcs

The upper tangent arc may be seen even when little or no main halo is visible. Where they do coincide at the top of the 22 degree halo the colours are often stronger than in the halo itself. The upper tangent arc is formed by pencil shaped ice crystals with their axes horizontal. Because it depends on the shape and orientation of the ice crystals it isn't always seen.

In the examples above there is a trace of the circular 22 degree halo. The tangent arc is more flattened and extends a little way either side of the halo.

This is typical of the appearance of the arc at medium solar altitudes where the arc is concave to the sun. When the sun is high in the sky the upper tangent arc wraps round to join up with the corresponding lower tangent arc. This forms the elliptical circumscribed halo.

The circumscribed halo lies a couple of degrees outside the 22 degree halo at the altitude of the sun, it coincides with the main halo above and below the sun.

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